feat(#1682): OpenCode session.idle + opencode-subset dialect + Claude parity — Slice 1b/c (#1930)

* feat(#1682): OpenCode session.idle + opencode-subset dialect + Claude parity — Slice 1b/c

- Plugin (.opencode/plugins/gsd-core.js): handle session.idle (↔ Claude Stop
  lifecycle point; no-op sentinel — state already persisted to .planning/).
  Completes the compaction/idle pair (#1914 shipped compaction).
- Declare hookEvents: 'opencode-subset' in the OpenCode descriptor — the
  reserved dialect now has a real consumer (no longer zero-consumer).
- host-integration.cts: add HOOK_EVENT_SURFACES + hookEventSurfaceFor() — the
  pure consumer that resolves a dialect to its host-fireable event surface.
  opencode-subset = session/tool/file subset with NO workflow-phase events
  (engine owns phase sequencing; ADR-1239 §OpenCode binding).
- Tests: hookEventSurfaceFor unit tests (claude/gemini/opencode-subset/null);
  plugin session.idle no-throw; compaction breadcrumb; opencode-subset surface
  parity vs the plugin's handlers (Claude parity).

* fix(#1682): don't declare hookEvents on opencode (hooksSurface:none invariant)

The repo invariant couples runtime.hookEvents to the managed settings.json hook
surface: hooksSurface:'none' runtimes (opencode — plugin owns hooks) must NOT
declare hookEvents. Declaring 'opencode-subset' there violated 3 capability-
registry invariants + 2 install-plan golden masters + opencode golden parity.

The opencode-subset dialect is still IMPLEMENTED — just not via the legacy
descriptor field: hookEventSurfaceFor() (host-integration.cts) is its consumer,
and the OpenCode plugin consumes the subset events at runtime (session.idle
added here; compaction shipped in #1914). Declaring it on the descriptor would
require weakening the hooksSurface:none ⇔ no-hookEvents invariant (flagged for
decision).

* fix(#1682): refresh opencode golden parity for plugins/gsd-core.js (session.idle)

* docs(changeset): OpenCode session.idle + opencode-subset dialect (#1682)

* docs(changeset): backfill PR #1930

.opencode/plugins/gsd-core.js

@@ -651,6 +651,18 @@ const GsdCorePlugin = async ({ directory } = {}) => {
         handleHookResult(r);
         return;
       }
+
+      // session.idle ↔ Claude Stop lifecycle point (#1682 Slice 1b/c).
+      // OpenCode fires session.idle when the run quiesces. GSD maps it to the
+      // Stop equivalent — the opencode-subset lifecycle peer of compaction
+      // (compaction preserves state across context-window summarization; idle
+      // marks end-of-turn). No-op sentinel today (GSD state is already
+      // persisted to .planning/), but it MUST be recognized so the declared
+      // opencode-subset surface is fully wired and a future Stop-class hook can
+      // attach without a plugin change.
+      if (event.type === "session.idle") {
+        return;
+      }
     },
   };
 };